Different optical devices such as polarizers and retarders have been developed to induce light polarization changes. Polarizers transmit only the component of input light with electric field vector oscillating parallel to the polarizer axis, while retarders introduce a phase shift between two orthogonal electric field components of the light. Polarizers and retardation plates are frequently used in combination to control laser intensity or polarization state for many different applications. For example, rotation of linear polarization can be achieved with half wave plate retardation. However, retardation plates are strongly dependent on radiation wavelength and have a small acceptance angle.
In recent decades liquid crystals (LC) have been thoroughly studied because of their interesting linear and nonlinear optical properties. Nematic liquid crystals (NLC) are particularly interesting because of their high birefrigence. In these liquids, the molecules tend to align parallel to each other. By placing the liquid crystal in a cell with a specially oriented glass surface, the liquid crystal molecules align with the glass surface orientation. This makes the NLC a uniaxial birefringent medium with an optical axis along the direction of alignment. This direction is commonly defined by a unit vector known as the xe2x80x9cdirectorxe2x80x9d. A linearly polarized beam passing through such a NLC polarized at an angle "PHgr" with respect to the director will generally have electric field components parallel to and perpendicular to the director. As the refractive index experienced by each of these components (ne and no respectively) are different, there will be a relative phase shift between these components and the NLC behaves like a wave plate.
If the opposite glass window of the cell has also an oriented surface, the molecules at that side of the cell will align in the orientation of that surface. If the two orientations are different, the orientation of the LC will gradually change, forming a helix. This is known as a twisted nematic liquid crystal (TNLC). Hiap Liew Ong characterized the optical properties of general twisted nematic liquid crystals (TNLC) (see Ong, H. L., J. Appl. Phys. 64, 614 (1988)).
The principle of fixed-angle rotation has been used in conjunction with electrical alignment of TNLCs in liquid crystal display technology. If linearly polarized light is incident on a TNLC cell with polarization vector parallel to (or perpendicular to) the director vector at the entrance window, the polarization remains linear, following the rotation of the director vector and hence exits with linear polarization parallel to (or perpendicular to) the director vector at the direction of the exit window. This process, known as xe2x80x9cadiabatic followingxe2x80x9d, only occurs if the pitch of the helix is much greater than the radiation wavelength inside the LC, otherwise the light becomes elliptically polarized. Here the pitch is P=(2xcfx80/xcex8)L, where xcex8 is the rotational angle and L is the physical length of propagation through the LC. Half-wave plates, fabricated out of birefringent crystals such as quartz, calcite, mica, etc., can also be used to continuously rotate linear polarization, but these are inherently strongly wavelength dependent and have a narrow field of view.
Simoni et al in U.S. Pat. No. 4,579,422 used a cholesteric liquid crystal in a device that can continuously rotate linear polarization through angles up to 45xc2x0 in response to an applied voltage, but this is a strongly wavelength dependent device.
The literature fails to disclose any optical device that can be used to continuously rotate linearly polarized light working simultaneously at all wavelengths, through any angle.
The first objective of the present invention is to provide an optical device, which can rotate the polarization of polychromatic light by any desired angle.
The second object of this invention is to provide an optical device, which can continuously rotate the polarization of polychromatic light to any desired angle.
The third objective of the present invention is to provide an optical device, which can rotate the polarization of light independent of the direction of propagation of the incident light.
The fourth objective of this invention is to provide an optical device, which can rotate the polarization of pulses of light, independent of the temporal duration of the pulses.
The fifth objective of this invention is to provide an optical device, which can rotate the polarization of polychromatic light and hence, when placed between broadband linear polarizers can control the power of any light source.
The sixth objective of this invention is to provide an optical device, which can rotate the polarization of polychromatic light, where that rotation may be switched on and off by means of an externally applied electrical voltage.
The preferred embodiment describes an optical device, comprising: a first light transparent window with an oriented inner surface; a second light transparent window with an oriented inner surface; twisted nematic liquid crystal disposed between said oriented surfaces of first and second windows; and, means for continuously rotating said windows whereby the direction of linear polarization of light transmitted through said device is altered.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated schematically in the accompanying drawings.